An amazing fraction--roughly half--of all the medicines prescribed today have a striking commonality. At the molecular level, they act on the same type of target: a serpentine protein that weaves seven times through the membrane that envelops the cell. External parts of each serpent serve as an antenna for molecular signals approaching the cell, and internal parts trigger the cell's responses to such cues, beginning with the activation of a signal processor called a G-protein. The serpents themselves are thus known as G-protein coupled receptors, or GPCRs.

As a group, GPCRs show far more versatility than any other class of cell-surface receptor. For instance, the natural molecules to which GPCRs respond range in size from neurotransmitters that are only a few times as massive as a single carbon atom all the way up to proteins 75 times larger than that. Moreover, GPCRs participate in just about every bodily function that sustains life, from heartbeat and digestion to breathing and brain activity. The drugs that target these receptors are equally diverse. The list includes blood pressure reducers (such as propranolol), stomach acid suppressors (such as ranitidine), bronchodilators (such as albuterol) and antidepressants (such as paroxetine). The disorders these medicines treat include hypertension, congestive heart failure, ulcer, asthma, anxiety, allergy, cancer, migraine and Parkinson's disease.